1. Field of the Invention
The present invention relates to a mixer circuit, and more particularly to an analogue mixer circuit which combines an intermediate frequency signal (hereinafter, called "IF signal") with a local signal (hereinafter, called "LO signal").
2. Description of the Related Art
FIG. 5 shows one example of the composition of a conventional mixer circuit 189. With reference to FIG. 5, the mixer IC (integrated circuit) 188 comprises one npn-type transistor 145. The collector of this transistor 145 is connected to an RE terminal 152, the base is connected to an input terminal 149, and the emitter is connected to a GND terminal 150. The base of the npn-type transistor 145 is connected to the junction point between a resistor 146 and a resistor 147. The other terminal of resistor 146 is connected to the collector of the npn-type transistor 145. The other terminal of resistor 147 is connected to the earth terminal (hereinafter, called "GND terminal") 150 of the mixer IC 188. The collector of the npn-type transistor 145 is connected to a power supply terminal 151 and furthermore, a resistor 148 is connected between the collector of the npn-type transistor 145 and the power supply terminal 151.
The power supply terminal 151 for supplying power to the mixer IC 188 is connected to a power supply 155, and the RF terminal 152 is connected via a capacitor 154 to an RF output terminal 164. Meanwhile, the input terminal 149 is connected via a capacitor 153 to the junction point between an inductor 157 and one terminal of a capacitor 160. The other terminal of the inductor 157 is connected to the junction point between an inductor 156 and one terminal of a capacitor 158. The other terminal of the inductor 156 is connected to an IF signal input terminal 162, and the other terminal of the capacitor 158 is earthed.
The other terminal of the capacitor 160 is connected to the junction point between the capacitor 159 and one terminal of the inductor 161, the other terminal of the capacitor 159 is connected to an LO signal input terminal 163, and the other terminal of the inductor 161 is earthed.
A first band-eliminating filter (BEF1) 186 is constituted by the capacitor 159, capacitor 160 and inductor 161, and a second band-eliminating filter (BEF2) 187 is constituted by the inductor 156, inductor 157 and capacitor 158.
A mixer circuit 189 is formed by the foregoing composition. In this mixer circuit 189, an IF signal is input from the IF signal input terminal 162 and it passes through the second band-eliminating filter 187. Furthermore, an LO signal is input from the LO signal input terminal 163, passes through the first band-eliminating filter 186 and is mixed with the IF signal as an input to the capacitor 153.
The first band-eliminating filter 186 and the second band-eliminating filter 187 are provided in order to isolate the IF signal and LO signal. Consequently, for the IF signal, the LO side appears to be in an open state due to the first band-eliminating filter 186, whilst for the LO signal, the IF side appears to be in an open state due to the second band-eliminating filter 187. By this means, the IF signal and LO signal are added together without one signal leaking in the circuitry of the other, and they pass through the capacitor 153 and are input to the input terminal 149 of the mixer IC 188.
The combined signal is amplified nonlinearly by the npn-type transistor 145 and it is then output from the RF terminal of the mixer IC 188, passed through the capacitor 154 and output from the RF output terminal 164.
Next, a further example of the related art is described with reference to FIG. 6. The composition of the mixer circuit 190 is disclosed in Japanese Unexamined Patent 63-258107. With reference to FIG. 6, in the mixer circuit 190, the collector of a transistor 165 is earthed via a capacitor 170. Therefore, this is similar to the high-frequency region being earthed. Furthermore, the npn-type transistor 165 operates nonlinearly, but the inductor 176 is connected via a capacitor 171 to the base of the npn-type transistor 165. The input terminal 180 of a local oscillator input from an external source is connected to the base of the npn-type transistor 165 via a wave filter circuit 177 and a capacitor 172. A capacitor 169 and a capacitor 173 are provided respectively between the base and emitter terminals, and between the emitter and GND terminals, of the npn-type transistor 165. The terminal 182 is a modulation signal input terminal. In addition, the emitter of the npn-type transistor 165 is connected to an output terminal 181 via a wave filter circuit 178.
By means of the foregoing composition, the emitter of the transistor 165 outputs a signal wherein a signal of frequency components input from the local oscillator and a signal of frequency components oscillating in the inductor 176 and capacitor 169, and capacitor 173, are combined with each other. Thereupon, the required signal only is extracted from a number of combined frequency component signals by means of the wave filter circuit (filter) 178.
FIG. 7 shows the results of a simulation of a conventional mixer circuit. In FIG. 7, the input frequency, f.sub.IF, of the IF signal is set to a constant 150 (MHz) at IF signal input power of P.sub.IFin =-35 (dBm). The output power P.sub.RF, of the RF signal when the input power of the LO signal, P.sub.LO, is swept is indicated by the symbol .quadrature., and the signal leakage from the LO terminal to the IF terminal is indicated by the symbol .omicron., and the signal leakage from the IF terminal to the LO terminal is indicated by the symbol .gradient., these values being plotted on a graph. In the graph, the LO signal frequency, f.sub.LO is 800 (MHz), and the RF signal frequency, f.sub.RF, is 950 MHz.
However, there have been the following problems with conventional mixer circuits.
The conventional mixer circuit 89 shown in FIG. 5 is composed such that the IF signal and LO signal are input via the same input terminal 149 of the mixer IC 188. Therefore, in order to isolate the IF signal and LO signal, it has been necessary to insert a first band-eliminating filter 186 and a second band-eliminating filter 187 comprising inductors and capacitors in the respective signal paths. Accordingly, there has been a problem in that this increases the installation surface area of the chip components forming the mixer circuit 18.
Moreover, in the mixer circuit 190 illustrated in FIG. 6, a large number of additional components, such as a bypass capacitor (capacitor 170), inductor 176, and a variable-voltage capacitor (varactor) 175, etc. are required, andthis requires aneven larger installation surface area. A further problem arises in that since the collector of the npn-type transistor 165 is earthed, insufficient conversion gain is obtained from the mixer circuit 190.